Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha’s rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp3 C-linked D–A compounds due to the arrest of intramolecular electronic communication between two triplet states (T1H and T1L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp2 C-linked D–A counterparts, RTP from one locally excited T1 state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.


■ INTRODUCTION
The efficient generation, controllable management, and ondemand migration of triplet excitons from higher-lying to lowerlying triplet excited states in organic molecules have combined to constitute the long-sought Holy Grail of room-temperature phosphorescence (RTP). However, the exciton spin-flip between well-separated singlet and triplet excited states is not allowed due to the violation of Winger's rule. 1,2 To tackle this apparent contradiction, ever-increasingly complex molecular design principles have been devised to modulate both the molecular structure and host−guest interactions. 3−5 Behind these is the control of the relative kinetics of facilitating intersystem crossing (ISC) 6,7 and suppressing nonradiative processes, 8−10 both of which are paramount to achieving efficient RTP. From the perspective of molecular design, introducing heteroatoms, such as N, 11,12 O, 13,14 P, 15−17 and S, 18−20 into metal/halogen-free aromatic compounds is a popular way to enhance the ISC efficiency through the introduction of n−π* states that allow for the spin-flip process to occur, following El-Sayed's rule. 21 Most recently, Marder and co-workers demonstrated that a (σ, B p ) → (π, B p ) transition localized on a three-coordinate boron atom could also accelerate ISC. 22 This new approach likewise relies on El-Sayed's rule to mediate state mixing and thus ISC from S 1 to T 1 . To sufficiently brighten the populated triplet excitons of organic compounds, it is also necessary to weaken nonradiative energy dissipation. To date, a plethora of methods, such as engineering molecular crystals, 11,14,23−25 employing a polymer matrix, 26−30 and constructing a rigid host environment, 31−34 have been exploited to achieve suppressed nonradiative decay. Following these principles, many RTP luminophores have been developed, showing moderate photoluminescence (PL) quantum yield (Φ PL ) and/or long phosphorescence lifetime (τ Ph ). These compounds have been exploited in diverse applications, such as imaging, 35−37 optoelectronics, 38−40 nonlinear optics, 41 and information encryption. 16,18,28,42 Multiple phosphorescence from a single component is an emergent phenomenon from the radiative decay of excitons from multiple accessible triplet excited states. Compounds that show multiple phosphorescence offer a handle into the study of triplet exciton dynamics. Due to Kasha's rule, 43 it remains a formidable challenge to enable multiple phosphorescence at room temperature, though there are well-studied systems that show anti-Kasha's fluorescence. 44,45 Thus, there have been only a few reports documenting anti-Kasha RTP in organic compounds. 46−48 For example, Wu and co-workers reported that heterocyclic stilbene derivatives exhibited room-temperature dual phosphorescence originating from both T 2 and T 1 states. 47 This results from the relatively large energy gap (>0.5 eV) between these two states, which inhibits the excitonic internal conversion (IC). Room-temperature dual phosphorescence has also been demonstrated in both systems where there is simultaneous emission from monomers and aggregates. 11,49 Recently, T 1 conformer-regulated multiple phosphorescence has been demonstrated by us, which renders the excited-state dynamics tunable via thermal activation. 20,50 However, a general principle to regulate multiple phosphorescence on demand remains elusive.
Here, we present a general multiple RTP design strategy for organic compounds by modulating the excitonic coupling between two T 1 states localized on the donor and acceptor via the degree of conjugation that the linker mediates between these two groups. We define the higher-lying and lower-lying T 1 states as T 1 H and T 1 L states, respectively. By comparing the photophysics of emitters containing sp 3 and sp 2 linkages (C vs CO), we found that dual phosphorescence relies on how strongly the two triplet excited states communicate. It was demonstrated in two examples that sp 3 C-linked compounds (Cz-C-PTZSO 2 and Cz-C-TRZ) (Figure 1a), where 9-phenylcarbazole (Cz-Ph) serves as the donor and 10-phenyl-10Hphenothiazine-5,5-dioxide (PTZSO 2 ) or 2,4,6-triphenyl-1,3,5triazine (TRZ) acts as the acceptor (Scheme S1a), could enable dual phosphorescence from two locally excited ( 3 LE) triplet states due to negligible excitonic coupling between donor and acceptor moieties at room temperature ( Figure 1b). In contrast, the sp 2 C-linked counterpart (Cz-CO-PTZSO 2 ) shows only one 3 LE emission at room temperature, together with aggregate phosphorescence in the crystal state, while no RTP was recorded in the other sp 2 C-linked compound (Cz-CO-TRZ).

■ RESULTS AND DISCUSSION
The synthesis of Cz-C-PTZSO 2 , Cz-CO-PTZSO 2 , Cz-C-TRZ, and Cz-CO-TRZ is outlined in Scheme S1. All samples were purified by repeated silica column chromatography, followed by temperature-gradient vacuum sublimation and finally recrystallization from toluene. The molecular structures and purity were validated by a combination of 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HRMS), melting point determination, elemental analysis (EA), and high-performance liquid chromatography (HPLC) (Figures S1−S32). We hypothesized that the IC between donor-and acceptorlocalized T 1 states could be slowed or even inhibited through the use of a nonconjugated sp 3 carbon atom to electronically isolate donor and acceptor groups (Cz-C-PTZSO 2 and Cz-C-TRZ), where multiple RTP would originate from both donor and acceptor subunits (Figure 1b). For sp 2 C-linked species (Cz-CO-PTZSO 2 and Cz-CO-TRZ), due to the stronger electronic communication between the donor and acceptor, the IC of excitons between the two low-lying triplet states is greatly enhanced, resulting in only phosphorescence from the T 1 L state. To substantiate this hypothesis, we first modeled the photophysical properties of these four compounds in the gas phase using density functional theory (DFT) at the PBE0 51 /6-31G(d,p) level. 52 From Figure 2a, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Cz-C-PTZSO 2 and Cz-C-TRZ are localized on the donor (Cz-Ph) and acceptors (PTZSO 2 and TRZ), showing a near-zero frontier molecular orbital (FMO) overlap. For Cz-CO-PTZSO 2 and Cz-CO-TRZ, the LUMO is delocalized across both the acceptor and the benzophenone core, resulting in deeper HOMO and LUMO levels ( Figure  S33). Time-dependent DFT calculations at the same level of theory demonstrate a relatively large energy gap between S 1 and T 1 states (ΔE ST ) in these compounds (Figure 2b), indicating that these compounds should not exhibit thermally activated delayed fluorescence. Given the presence of multiple triplet state energy levels serving as upper receiver states and moderate spin−orbital coupling constants ( Figure S34), RTP should be theoretically favored as the dominant radiative decay process. The T 1 and T 2 energies of Cz-C-PTZSO 2 and Cz-C-TRZ are almost degenerated with those of the T 1 energies of the isolated donor, Cz-Ph, and acceptors, PTZSO 2 and TRZ ( Figure S35b). For Cz-CO-PTZSO 2 and Cz-CO-TRZ, similar T 1 energies imply that both states have the same character, which is close to that of the LE T 1 state of the carbazole-benzophenone (Cz-BP) subunit ( Figure S35b). Furthermore, the excitonic coupling between T 1 and T 2 was investigated based on the hole−electron distribution analysis using the Multiwfn program. 53 Cz-C-PTZSO 2 and Cz-C-TRZ both show decoupled T 1 and T 2 states (Figure 2c), where the hole (green) and electron (blue) distributions are identical to those of the donor (as the T 1 state for Cz-C-PTZSO 2 ; as the T 2 state for Cz-C-TRZ) and the acceptor (as the T 2 state for Cz-C-PTZSO 2 ; as the T 1 state for Cz-C-TRZ) ( Figure S35c). Considering that the T 1 and T 2 states are localized on the donor and acceptor subunits and possess almost identical triplet energies with the T 1 energies of the isolated subunits, here we redefine the T 2 and T 1 states as T 1 H and T 1 L states, respectively. According to Fermi's golden rule, 54 the reduced vibronic coupling between T 1 H and T 1 L states should also favor a slow IC, enabling emission from both the donor and acceptor groups. However, Cz-CO-PTZSO 2 and Cz-CO-TRZ exhibit strongly coupled triplet excited states due to the overlap of the hole and electron densities as well as the small energy gap between T 1 H and T 1 L states; thus, there should be rapid IC between T 1 H and T 1 L states. T 1 spin density distributions reveal that in Cz-C-PTZSO 2 and Cz-C-TRZ, they are localized on Cz-Ph (Figure 2d), while for Cz-CO-PTZSO 2 and Cz-CO-TRZ, the spin densities are relatively delocalized (Figure 2d), similar to the hole−electron density distribution analysis (Figure 2c). Calculated T 1 energies at the optimized T 1 geometry are 2.86, 2.03, 2.86, and 1.98 eV for Cz-C-PTZSO 2 , Cz-CO-PTZSO 2 , Cz-C-TRZ, and Cz-CO-TRZ, respectively.
The energies of FMOs were inferred from the oxidation and reduction potentials (E ox /E red ), obtained using cyclic voltammetry and differential pulse voltammetry (DPV) in deaerated DMF with 0. We next investigated the photophysical properties of the Cz-C-PTZSO 2 and Cz-CO-PTZSO 2 crystals, which were isolated from a toluene solution. The photophysical data are summarized in Table S2. The steady-state PL spectrum of Cz-C-PTZSO 2 in air at 298 K shows a structured LE emission profile with peak maxima, λ PL , at 403 and 426 nm, alongside a broad emission tail (Figure 4a). The corresponding PL lifetime, τ PL , is ∼3.5 ns, recorded at both 403 and 426 nm ( Figure S36a). When placed under vacuum, the broad PL band was enhanced with λ PL at 403 and 426 nm, indicating the most likely involvement of triplet excited states. Time-gated PL measurements (3 ms delay) detected the RTP spectrum of Cz-C-PTZSO 2 , where dual  (Figure 4d), respectively. Figure 4b shows RTP spectra collected at different delay times, which demonstrate that the RTP at 470 nm decays faster than that at 530 nm, reflecting the results of the RTP lifetime measurements (Figure 4d). These dual emission bands match well with the enhanced LE emission and the emission tail presented in the steady-state PL spectra in vacuum. The phosphorescence spectrum recorded at 80 K blue-shifts to 475 nm and is dominated by a structured blue emission band, whose low-temperature phosphorescence (LTP) lifetime is prolonged to 836.7 ms and longer than that recorded at 530 nm (τ Ph = 536.6 ms) ( Figure S37a). In line with the proposed multiple phosphorescence model (Figure 1b), we ascribe the dual RTP emission to result from phosphorescence from each of the Cz-Ph and PTZSO 2 moieties. To verify these assignments, we measured the phosphorescence spectra of Cz-Ph and PTZSO 2 at 77 K ( Figures S38 and S39a). It was found that one of the two RTP emission bands of Cz-C-PTZSO 2 matches the phosphorescence of the Cz-Ph monomers ( Figure S38), while the lowenergy RTP band is red-shifted compared to the phosphorescence of PTZSO 2 ( Figure S39a); rather, this phosphorescence aligns with the phosphorescence of PTZSO 2 aggregates ( Figure  S39a). Time-gated phosphorescence measurements of Cz-C-PTZSO 2 recorded in dilute 2-MeTHF glass at 77 K reveal the evolving structured phosphorescence spectra as a function of time ( Figure S40a). Distinct from the phosphorescence behavior observed in the crystal, in 2-MeTHF glass, dual phosphorescence is observed originating from LE phosphorescence of the Cz-Ph donor (T 1 L ) and PTZSO 2 acceptor (T 1 H ), which aligns with the DFT calculations.
The behavior of Cz-CO-PTZSO 2 is different. The steadystate PL spectra in air and vacuum at 298 K are similar (λ PL = 440 nm) (Figure 4a), with a PL lifetime of 0.7 ns ( Figure S36b). Although dual RTP behavior was recorded, it differs from that of Cz-C-PTZSO 2 . The blue RTP emission band from Cz-Ph is absent in Cz-CO-PTZSO 2 , but a new structured RTP emission band is present at λ PL of 572 nm. At 80 K, the phosphorescence emission blue-shifts to the green emission range, whose lowtemperature LTP lifetime is 89.8 ms ( Figure S37b). We assigned dual phosphorescence to originate from dual emission from the monomer and the aggregates, which is consistent with the recorded PL spectra of the model systems. The computed hole− electron distribution analysis revealed that the phosphorescence of Cz-CO-PTZSO 2 results from LE T 1 emission from Cz-BP (Figure 2c). Therefore, we measured the phosphorescence spectrum of Cz-BP at 77 K ( Figure S39b), which coincides with the green RTP emission band of Cz-CO-PTZSO 2 . We can therefore assign this band to phosphorescence from the Cz-BP moiety. To determine the origin of the orange RTP emission, we collected RTP spectra at various delay times. The results revealed that the RTP at 520 nm decays much faster than the RTP at 572 nm (Figure 4c), with corresponding τ Ph of 12.3 ms and 244.3 ms, respectively (Figure 4d). By closer inspection of the phosphorescence spectra of dilute samples in 2-MeTHF glass at 77 K, we found that the emission centered at 572 nm vanished ( Figure S41b), implying that the lower-energy RTP of Cz-CO-PTZSO 2 crystals originates from aggregates. As a result, we can conclude that dual phosphorescence of Cz-CO-PTZSO 2 belongs to the mixture of T 1 emission of the monomer and aggregates rather than T 1 H and T 1 L emissions from different subunits. The Φ PL values of Cz-C-PTZSO 2 and Cz-CO-PTZSO 2 at 298 K in air are 5.9 and 4.6%, respectively, which improve to 8.2 and 4.7% under a N 2 atmosphere, respectively (Table S2). Therefore, the calculated RTP Φ PL values of Cz-C-PTZSO 2 and Cz-CO-PTZSO 2 in N 2 are approximately 4% and 0.1%, respectively (Figure 4e). The weaker RTP of Cz-CO-PTZSO 2 may be caused by the presence of increased nonradiative channels. Figure 4f shows the RTP afterglows of Cz-C-PTZSO 2 and Cz-CO-PTZSO 2 in vacuum.
Further, temperature-dependent PL measurements were conducted to interrogate the origin of the multiple phosphorescence. For Cz-C-PTZSO 2 , the apparent blue shift of the phosphorescence emission occurs as the temperature decreases (Figure 5a), indicating that phosphorescence from Cz-Ph gradually dominates the spectral profile at lower temperatures. At 80 K, blue-shifted and structured phosphorescence spectra at different time-gated windows were observed, which confirmed that phosphorescence from T 1 H and T 1 L states of Cz-C-PTZSO 2 is dominated by the LE emission from the Cz-Ph and PTZSO 2 groups (Figure 5b), respectively, as these spectra align with those of the phosphorescence spectra of Cz-Ph monomers and PTZSO 2 aggregates (Figures S38 and S39a) (Figure 5c). The images of Cz-C-PTZSO 2 also evidence that the afterglow gradually varies from green to blue (Figure 5f). The approximated calculated slow IC rate between T 1 H and T 1 L indicates that there is weak communication between the two triplet excited states associated with the donor and acceptor subunits (Table S3), which explains the occurrence of T 1 H phosphorescence. A similar scenario was observed in Cz-CO-PTZSO 2 , where temperature-dependent phosphorescence blue-shifts as a function of decreasing temperature (Figure 5d). However, at 80 K, phosphorescence from Cz-BP dominates the emission, independent of the time-gated window (Figure 5e). The CIE diagram and images of afterglows document the unchanged phosphorescence color (Figure 5c,f) over time, indicating that the emission results from phosphorescence originating from the Cz-BP moiety of Cz-CO-PTZSO 2 (i.e., from the T 1 state). The ΔE ST of Cz-C-PTZSO 2 and Cz-CO-PTZSO 2 in dilute samples in 2-MeTHF glass at 77 K, estimated from the difference in energy between the onsets of the prompt and delayed emission spectra, are 0.28 and 0.41 eV, respectively ( Figure S41); it should be noted that ΔE ST of Cz-C-PTZSO 2 is in fact the energy gap between S 1 and T 1 H states (Cz-Ph). Next, the photophysical properties of the Cz-C-TRZ and Cz-CO-TRZ crystals were investigated. There is no obvious change to the steady-state PL spectra of Cz-C-TRZ in air and vacuum, centered at 448 nm (Figure 6a), whose PL lifetime is 5.1 ns measured at 298 K in air ( Figure S42a). At 298 K, time-gated PL measurements detected two well-separated phosphorescence bands centered at 460 and 575 nm, with associated lifetimes of 7.1 and 197 ms, respectively (Figure 6b). However, at 80 K, only one green phosphorescence band at 515 nm (τ Ph = 400 ms, Figure S43) was observed (Figure 6a). Compared to the phosphorescence spectra of each of Cz-Ph and TRZ ( Figures  S38 and S39c), we conclude that blue RTP and green LTP originate from states localized on Cz-Ph and TRZ, denoted as T 1 H and T 1 L , respectively, similar to the theoretical prediction (Figure 2c). Similarly, variation of the relative intensity of the structured phosphorescence spectrum of Cz-C-TRZ in 2-MeTHF glass recorded at 77 K at different time-gated windows also evidences contributions from the emission from both the T 1 H and T 1 L states associated with the Cz-Ph and TRZ moieties, respectively ( Figure S40b). Given that the TRZ moiety is prone to forming π−π stacked assemblies, we postulated that the orange RTP likely originates from molecular aggregates. To better elucidate the origin of the multiple triplet excited states, phosphorescence spectra were recorded at different time-gated windows. As shown in Figure 6c, at early time, blue RTP is dominant. Then, emission from aggregates dominates the overall RTP emission, accompanied by a small contribution from the TRZ phosphorescence at around 500 nm. The RTP afterglow color of Cz-C-TRZ varies from blue to orange ( Figure  6f). Temperature-dependent phosphorescence measurements also documented that multiple emissive triplet states exist in the Cz-C-TRZ crystals (Figure 6d). At 80 K, LTP emission negligibly changes with time (Figure 6e), reflected by the observed unchanged LTP afterglow (Figure 6f). To further confirm that the orange RTP is from aggregates, the RTP emission of Cz-C-TRZ crystals after grinding was recorded. We found that the overall RTP intensity was weakened, where the blue RTP intensity ratio was increased compared to the orange RTP due to the disrupted intermolecular interactions in the aggregates ( Figure S44). We prepared Cz-C-TRZ crystals via slow sublimation, which we hypothesized would produce samples with increased π−π stacking. These samples of Cz-C-TRZ show stronger orange RTP and negligible blue RTP ( Figure S45a,b,e). The RTP lifetime and RTP Φ PL of Cz-C-TRZ crystals that were acquired via temperature-gradient vacuum sublimation increase to 540.3 ms and 3.0%, respectively ( Figure   S46a, Table S4) compared to the respective values of the Cz-C-TRZ crystals (τ Ph = 190.7 ms, RTP Φ PL = 0.6%) isolated from toluene. At 80 K, green LTP from TRZ centered at 500 nm was also observed, likely due to the suppression of nonradiative processes ( Figure S45c), whose lifetime is prolonged to 820.8 ms ( Figure S46b). For Cz-CO-TRZ, steady-state PL spectra show an unstructured emission centered at 486 nm at room temperature (Figure 6a), whose overall Φ PL and lifetime in air are 13.9% and 2.4 ns, respectively (Table S2, Figure S42b). Unfortunately, no RTP was observed, probably resulting from the strong nonradiative decay. A broad and structured LTP profile was recorded at 80 K (Figure 6a). Time-gated PL measurements reflect that the LTP results from LE emission contributions from each of Cz-Ph, Cz-BP, and TRZ ( Figure  S47) based on the cross-comparison with the LTP from each of these isolated compounds ( Figures S38 and S39b,c). The lowtemperature multiple phosphorescence bands may result from restricted molecular conformations that lead to weak electronic coupling at low temperature. 20,57 We then investigated the single-crystal structures of Cz-C-PTZSO 2 , Cz-CO-PTZSO 2 , and Cz-C-TRZ to determine what influence if any intermolecular interactions would have on the observed multiple phosphorescence. For Cz-C-PTZSO 2 , strong CH···O and O···π intermolecular interactions exist between PTZSO 2 moieties (Figure 7a), which is consistent with the assignment that the T 1 L emission of Cz-C-PTZSO 2 at room temperature is almost identical to that from the acceptor PTZSO 2 aggregates. No intermolecular interactions were found between the Cz-Ph fragments, implying that the emission from the T 1 H state is from monomeric Cz-Ph. For Cz-CO-PTZSO 2 , intermolecular CH···π, CH···O, and π···π interactions are present (Figure 7b), which is consistent with the orange RTP of Cz-CO-PTZSO 2 originating from Cz-CO-PTZSO 2 aggregates, in addition to the green RTP from the Cz-BP moiety. For Cz-C-TRZ, π···π intermolecular interactions exist between adjacent TRZ fragments (Figure 7c), which result in excitonic splitting and thus a lower T 1 energy (orange RTP). Similar to Cz-C-PTZSO 2 , no apparent intermolecular interactions exist between the adjacent Cz-Ph groups, resulting in the occurrence of RTP from Cz-Ph in Cz-C-TRZ.
In addition, intermolecular interactions were studied in silico by employing noncovalent interaction calculations 58 implemented in the Multiwfn program. 53 The geometries of the emitters are those in the single crystals. As shown in Figure 8a, green spikes of the reduced density gradient (RDG), where the electron density is near zero, confirm the existence of intense van der Waals interactions, which can suppress molecular motions resulting in reduced nonradiative decay in the solid state. The RDG isosurfaces of Cz-C-PTZSO 2 reveal noncovalent interactions between the PTZSO 2 groups (Figure 8b) and negligible interactions between Cz-Ph groups, similar to the conclusions drawn from the single-crystal analysis. For Cz-CO-PTZSO 2 , evident noncovalent interactions are distributed across the entirety of the molecules, implicating that the orange RTP belongs to Cz-CO-PTZSO 2 aggregates. As anticipated, π−π interactions are present in the TRZ moiety of Cz-C-TRZ and CH···π and π····π interactions exist between Cz-Ph and TRZ. As a result, molecular packing of Cz-C-TRZ influences the occurrence of dual RTP, where condensed molecular packing results in strong Cz-Ph and TRZ interactions and thus efficient triplet−triplet energy transfer (TTET); that is, only orange RTP can be observed in this instance.

■ CONCLUSIONS
Herein, we have reported a general multiple RTP design principle and validated it using four organic compounds (Cz-C-PTZSO 2 , Cz-CO-PTZSO 2 , Cz-C-TRZ, and Cz-CO-TRZ). DFT calculations demonstrated that for sp 3 C-linked com-  pounds, dual RTP could be enabled from both the donor and acceptor fragments due to the absence of excitonic coupling between the T 1 H and T 1 L triplet excited states. However, the excitonic coupling channel of the two states is enabled by employing an sp 2 C as a linker between the donor and acceptor moieties, resulting in only one 3 LE emission. Photophysical investigations substantiated this design principle. For Cz-C-PTZSO 2 and Cz-C-TRZ crystals, dual RTP emissions from T 1 H and T 1 L states were recorded, which are LE emissions from the donor and acceptor fragments, respectively, together with the acceptor aggregate phosphorescence. For the Cz-CO-PTZSO 2 crystal, only one LE T 1 -dominated RTP was observed due to the strong excitonic coupling between the donor and acceptor; benefitting from the multiple intermolecular interactions, molecular aggregate phosphorescence was also activated. However, for Cz-CO-TRZ, there is no RTP, probably due to significant nonradiative decay. Single-crystal and RDG analyses demonstrated that noncovalent interactions influence multiple RTP behavior and TTET. This work provides insights for the design of organic systems that show dual phosphorescence and discloses an effective strategy for designing color-variable RTP that can potentially be exploited in a range of applications.

Accession Codes
CCDC 2203532−2203534 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.